Boron carbide composition and method of making the same



Patented May 14, 1940 UNITED STATES.

n c comosmon AND sum METHOD OF MAKING THE John A. Boyer, Falls, N. Y or to The Carborundnm Company, Niagara Falls, N. Y., a corporation of Delaware No Drawing. Application May 9, 1938, Serial No. 206,839

This invention relates to metallic alloys in which boron carbide separates as a crystalline phase from a melt containing one or more metals. The preparation of such alloys is described and claimed in my copending application, Serial No. 12,589, filed March 23, 1935, now Patent 2,124,538, granted July 26, 1938, of which the present application is a continuation in part.

My invention further relates to articles made from such alloys and has as its object the provision of articles formed from such alloys which have metallic properties and in which one or more of the constituents present is characterized by extreme hardness.

Boron carbide is one of the hardest materials known, and its hardness even exceeds that of silicon carbide and fused alumina, which are used extensively as abrasives. Although fused boron carbide is relatively tough in comparison with these abrasives, it does not possess the mechanical properties ordinarily associated with metals, and in comparison with such metallic materials as copper, nickel, and the common alloys of these metals, it is extremely brittle. Most metallic alloys, on the other hand, do not contain any constituent whose hardness even approaches that of boron carbide.

I have found, as set forth in my copending application above referred to, that boron carbide will alloy readily with most common metals, and that the alloys produced contain an extremely hard constituent either interspersed with metal or embedded in a metallic matrix. In order to produce such alloys, mixtures of boron carbide and the metal used as the alloying ingredient are heated to a temperature sufilciently high to reduce the mass to a state of fluidity or fusion. The temperature required is comparatively high, and is usually in the vicinity of 2000 C. Upon cooling, the boron carbide separates as a distinct crystalline phase, and the metal solidifies in the interstices between the boron carbide crystals. If the quantity of metal present is sufficient to form a continuous matrix, the boron carbide occurs as small crystals, usually microscopic in size, distributed throughout a matrix of metal. A material of this character thus combines the hardness of the boron carbide crystals with the toughness and ductility of the metal forming the matrix.

In the preparation of such alloys, I have found that copper, nickel, cobalt and iron, 1. e., those metals having consecutive atomic numbers from 26 to 29 inclusive, will produce alloys of the type herein described. These metals, presumably because'of their electronic structures, have closely related physical properties. The metal forming the matrix can, of course, be toughened or hard-.

ened by any of the common alloying ingredients ordinarily used for such purposes. For example,

if copper forms the base metal of the matrix, the copper can be alloyed with a small proportion of tin, aluminum, or nickel, or it a nickel matrix is employed, the nickel can be alloyed with copper or aluminum. All of these metals, it will be noted, have melting points which are lower than that of boron carbide.

When a charge containing about equal parts by weight of boron carbide and metal is heated until the mixture becomes fluid, the boron carbide apparently dissolves in the metal, and upon cooling, crystallizes as small crystals dispersed throughout the metal. This product has a structure similar to that of many bearing metals, in which microscopic crystals of an alloy constituent sufilciently hard to resist wear, are embedded in a ductile matrix which imparts toughness, resilience and workability to the alloy. The boron carbide is, however, much harder and more resistant to wear than any metallographic constituent found in the usual bearing metals.

If the metal content of my alloy is increased to about per cent by weight, the material can be cut and worked much more readily than can an alloy having a higher boron carbide content. In considering the metal content of these alloys, however, it should be remembered that the percentage of metal by volume is very much less than the percentage by weight, owing to the great difference in specific gravities of the boron carbide and the metals used as alloying agents. For example, in an alloy containing 50 per cent copper by weight, the metal is the continuous constituent, but it forms only a comparatively thin network between the boron carbide crystals. In an alloy containing 80 per cent copper by weight, the boron carbide and the copper are in approximately equal proportions by volume, and the boron carbide crystals are scattered uniformly throughout the copper matrix.

My metal-boron carbide alloys may be prepared by heating the two materials to a high temperature in a graphite crucible. If, however, a molten alloy containing iron, cobalt or nickel is allowed to remain in contact with the graphite crucible for any appreciable time, the melt absorbs carbon, and upon cooling, the mass is found to be permeated by a considerable number of graphite flakes, so that the material has a graphitic structure very similar to that of grey cast iron. Such a material can be readily cut and worked, although it contains a high proportion by volume of boron carbide.

For purposes where toughness is a. primary consideration, however, the graphite flakes can be eliminated by taking special precautions to prevent the absorption of carbon by the melt, and it is even possible to oxidize some of the carbon present so as to produce an alloy containing such anexcess of boron that a boride separate crystalline constituents embedded in the metal matrix which usually difler slightly from each other in color. It will 'be realized that the exact identification of metallographic constituents in ternary alloys presents considerable difficulty, but as such alloys as these are made under conditions which partially oxidize the carbon of the boron carbide, the constituents embedded in the matrix are presumably boron carbide and a second constituent due to the presence of excess boron in the melt, the second constituent probably being a boride of the metal forming the matrix. The boride constituent is extremely hard, and can be polished with 400 grit silicon carbide paper on a rotating lap without scratching.

The absorption of carbon from the crucible by the alloys can be minimized or prevented by very rapid heating or by employing a resistor placed above the melt as fully described in my copending application above referred to. Crucibles of magnesia or carbon crucibles lined with magnesia can also be used and the last mentioned procedure can be employed in conjunction with crucibles of this type, or if the alloy is to be made in bulk without regard to the shape of the final product, the unmelted portion of the charge can form the container for the molten alloy. The heating of the charge from above eliminates the necessity of passing the heat through the container, and the crucible holding the charge can be maintained at a very much lower temperature than when all of the heat used for melting the alloy is passed through the container itself. Heating can also be carried out in a high frequency induction furnace. If a conductive carbon crucible is used, the crucible should be lined with a refractory oxide (as for example magnesium oxide) or the heating carried out very rapidly to prevent contamination from carbon. An overhead resistor can also be heated inductively if desired.

Boron carbide substantially free from excess carbon or graphite, which may be purchased on the open market, can be used as a raw material with the chosen metal in making the alloys. It is also possible to produce the alloys by the direct reduction with carbon of a mixture of the metallic oxide and boric oxide in the proper proportions, using a process substantially like that used for the production of pure boron carbide. The alloys in crude or lump form can also be made by heating a mixture of borio oxide, the oxide of the metal desired in the alloy, and carbon in the proper stoichiometric proportions in a furnace of the overhead resistor type, such as one like that described in copending application Serial No. 12,566, filed March 23, 1935, using themreduced portion of the charge, or preferably an unconverted mixture of boric oxide and carbon as a container for the melt. The mixture of oxides and carbon can also be reduced by this method while contained in a crucible or mold.

In forming the articles which are the particular object of my invention, boron carbide-metal alloys of the type herein described are crushed to reduce them to granular form or to a powder. The crushed alloy is then rebonded by sintering or fusing at a relatively low temperature, as for example, from 800 to 1400 C. depending upon the metal used as a bond. The rebonding of the crushed boron carbide-metal alloys is greatly simasoaase p'lifled by their property of passing through a pasty stage at temperatures above the melting point of the metal constituent instead of undergoing a sudden transformation from solid to liquid. Accordingly, articles formed from the alloys may be heated to temperatures above the melting point of the metal constituent (e. 8.. about 1300' C. in the case of copper) without materially altering their shape. Moreover, when pressed in molds at high temperatures this pasty characteristic renders less the likelihood that a portion of the alloywill be squeezed out of the mold.

Where it is desired, additional metal, which may be the same as that of the alloy or different, may be added in powdered form to the crushed alloy before rebonding, thus allowing for the production of various types of bond. Furthermore, additional non-metallic material may be added before. This material may be more boron carbide or may be other-hard materials such as diamond, silicon carbide, fused alumina, and the like. When additional amounts of these hard materials are added, the metal forming the matrix in which boron,carbide particles have crystallized will also serve as a bonding metal for the added hard materials.

Pressure can be applied to the mixture of granulated or powdered alloy and the additional metal and/or hard material prior to sintering to preform the article and if desired, or necessary, pressure may be applied during the sintering. The time required for sintering will depend upon the temperature and the pressures, if any, used.

The products obtained by breaking up and rebonding these boron carbide-metal alloys are suitable for use as abrasive articles for which purpose they may be formed as wheels, or in other desired shapes and as wear-resistant articles such as bearings.

The following specific examples are illustrative of the application of my invention, but are not to be considered limiting since changes in proportions and ingredients may be made depending upon the use for which the articles are desired. It will be understood that metals of the group, copper, iron, nickel and cobalt other than the ones specified may be used if desired.

Example I.A wear-resistant article suitable for use as a bearing is prepared by finely crush ing an alloy of copper and boron carbide containing 50 per cent copper and rebonding the crushed alloy by sintering the same in the shape desired. 1

Example II .A wear-resistant composition which is suitable for forming into abrasive articles or bearing surfaces is formed by crushing a boron carbide-copper alloy containing 50 per cent copper and mixing with 75 parts of the alloy, 22.5 parts of copper powder and 2.5 parts of powdered tin. When thoroughly mixed, pressure and heat are applied to sinter the mass into the desired shape.

Example III.-Another example of a composition suitable for use in forming abrasive articles or bearings is obtained by admixing parts of a crushed boron carbide-nickel alloy containing 40 per cent nickel with-20 parts of copper powder. The mixture is then formed in the desired shape and sintered.

Example IV.Articles having greatly increased abrasive properties are obtained by admixing 80 parts of a crushed copper-boron carbide alloy containing 70 per cent copper with 10 parts of copper powder and 10 parts of diamond particles ranging in size from 80 to 140 mesh and then molding under pressure and sintering the mix-- ture.

Example V.--An abrasive article of somewhat diflerent composition is obtained by admixing 70 parts of boron carbide-copper alloy containing per cent copper with 5 parts of tin powder, 15 parts of copper powder and 10 parts of diamond particles ranging in size from 80 to 140 mesh, pressing the mixture in the desired shape and thereafter sintering it.

In the case of articles which are designed to be used'as bearings, the face of the alloy should, of course, be lapped and polished to prevent any abrading action on the part of the crystals of boron carbide or other hard material held in the metal.

Articles made according to my invention differ materially from those obtained by merely bonding boron carbide with a metal since the boron carbide and metal are, according to my invention, associated together in the form of an alloy. In a boron carbide-metal alloy, the boron carbide forms an integral part of the material, and even when the alloy is crushed the boron carbide crystals do not separate from the metal. In i'ebonding a crushed boron carbide-metal alloy with additional metal, the added metal alloys readily'with the metal present in the original alloy to form an integral alloyed mass.

On the other hand when boron carbide alone is bonded with added metal under the conditions which obtain in the usual sintering or bonding procedures, there is no alloying of the boron carbide with the metal bond. The boron carbide particles in the mass or article are the original particles introduced into the mix and they are not crystallized from the melt, but are merely retained mechanically by the metal bond.

Having thus described my invention, I claim:

1. The method of making a metal bonded article containing-boron carbide which comprises alloying the boron carbide by a fusion process with suflicient metal to form, upon fusion and solidification an alloy, the metal components of the alloy having lower melting points than boron carbide, in which the metal forms a substantially continuous matrix for the deposited crystals of boron carbide, disintegrating the alloy so formed, and thereafter bonding the disintegrated particles at a lower temperature than that used in forming the original alloy,

2. The method of making a metal bonded article containing boron carbide which comprises alloying the boron carbide by a fusion process with suficlent metal to form, upon fusion and solidification an alloy, the metal components of the alloy having lower melting points than boron carbide, in which the metal forms a substantially continuous matrix for the deposited crystals of boron carbide, disintegrating the alloy so formed,

admixing therewith additional finely divided 5 metal and thereafter bonding the mixture of additional metal and particles of the disintegrated alloy by heating at a lower temperature than that used in forming the original alloy.

3. The method of making a metal bonded ar- I ticle containing boron carbide which comprises the alloy having lower melting points than boron carbide, in which the metal forms a substantially continuous matrix for the deposited crystals of boron carbide, disintegrating the alloy so formed, admixing therewith particles of hard non-metallic material and thereafter bonding the particles of hard material and disintegrated alloy by heating at a lower temperature than that used in forming the original alloy.

4. The method of making a metal bonded article containing boron carbide which comprises alloying the boron carbide by a fusion process with suflicient metal to form, upon fusion and solidification an alloy, the metal components of the alloy having lower melting points than boron carbide, in which the metal forms a substantially continuous matrix for the deposited crystals of boron carbide, disintegrating the alloy so formed, admixing therewith additional boron carbide and thereafter bonding the mixture of boron carbide and particles of disintegrated alloy at a lower temperature than that used in forming the original alloy.

5. As a novel article of manufacture, a metal bonded article containing particles of a separately formed fused boron carbide-metal alloy, the metal of the alloy being present in such an amount as to form a substantially continuous matrix for the boron carbide and the metal components of the alloy having lower melting points than boron carbide.

6. As a novel article of manufacture, a metal bonded article containing a hard non-metallic material bonded with a separately formed fused alloy of boron carbide and a metal, the metal being present in such an amount as to form a substantially continuous matrix for the boron carbide and the metal components of the alloy having lower melting points than boron carbide.

7. As a novel article of manufacture, an article in which particles of a separaeely formed fused boron carbide-metal alloy, the metal components of which alloy have lower melting points than boron carbide, are bonded together by sintering to form a porous, hard, wear-resistant body, the metal of the alloy being present in such an amount as to form a substantially continuous matrix for, the boron carbide.

8. A novel article comprising particles of hard abrasive material held in a porous matrix of a separately formed fused boron carbide-metal alloy, the metal of the alloy being present in such an amount as to form a substantially continuous matrix for the boron carbide and the metal components of the alloy having lower melting points than boron carbide.

9. An article of manufacture as set forth in claim 5 in which the metal of the alloy is selected from the group of metals having atomic numbers from 26 to 29, inclusive.

10. An article of manufacture. as set forth in claim 5 in which the metal of the alloy is selected from the group of metals having atomic numbers from 26 to 29, inclusive.

11. An article of manufacture as set forth in claim 5 in which the metal alloyed with boron.

carbide is copper.

12. An article of manufacture as set forth in claim 5 in which the metal alloyed with boron carbide is a member of the iron group.

JOHN A. BOYER. 

